Answers

2014-09-17T23:36:56+08:00
In 1 min  pipe A can fill 1/20 part of a tank
in 1 min pipe B can fill 1/30 part  of a tank
in 1 min working together the two pipes can fill  1/20 +1/30
part a tank, which is      5/60 =  1/12
thus they will take    12/1  =    12 mins to fill the tank
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